The Nets got back into this Eastern Conference semifinal by beating the Heat in Game 3. But if they don’t follow it up with another win in Game 4 Monday, it won’t matter.

“We can’t ease into the games anymore,” Paul Pierce said after the Nets practiced Sunday. “They’re too good and they’re too experienced. We got to be desperate from here on out. Every game has to be treated like a Game 7.”

“We go home [Monday] night and play them in Game 4, it’s got to be treated like a Game 7. When we get down [to Miami] for Game 5, it’s got to be treated like a Game 7. We’re playing for our playoff lives. We have to be in that desperate mode.”

They were certainly playing with an air of desperation in Game 3, when the Nets used a strong second half to break the game open and hand the two-time defending champions their first loss of the postseason, giving the Nets new life after dropping the opening two games in Miami.

But their hopes of making this a true series and giving the Heat a real scare, as opposed to an easy trip into the Eastern Conference Finals, hinge on the outcome on Game 4. With a win, the Nets head back to South Beach for Game 5 Wednesday in a best-of-three situation. With a loss, they’ll be faced with the daunting task of having to knock off the champs three times in a row to dethrone them.

Miami Heats Lebron James (R) and Chris Anderson (L) defend against Paul Pierce (C) of the Brooklyn Nets during Game 3.Photo: AFP/Getty Images

“We need to tie this thing up and head back to Miami,” Deron Williams said. “We don’t want to go down 3-1, just like we didn’t want to go down 3-0.”

The Nets avoided going down 3-0, and facing certain defeat, thanks to a pair of adjustments they made prior to Game 2 that have proved quite effective. After Miami scored 52 points in the paint in Game 1, making nearly as many field goals inside (26) as the Nets attempted (28), the Heat combined to score 60 points in the paint in Games 2 and 3 as the Nets did a much better job of walling off the paint and forcing Miami to become a jump-shooting team.

And, in less of an adjustment than a change in production, the Nets got a lot more help from their bench. In Game 3, Andray Blatche and Mirza Teletovic combined to produce 27 points and 16 rebounds, outscoring Miami’s entire second unit (25 points) by themselves.

“[Jason] Kidd came in and made some adjustments,” Blatche said. “He said he wanted us to either drive the ball and kick it, keep it moving, don’t settle, and that is what we try to do, not settle and be aggressive.”

With the Nets knowing their season was on the line Saturday, they clearly were the team with more energy from start to finish. The only player on the Heat matching their effort level was LeBron James in the first quarter, when he poured in 16 points on 6-for-7 shooting.

But the Nets know Miami is going to play with a different sense of urgency in Game 4, knowing a win would allow them to close things out back on the shores of Biscayne Bay on Wednesday.

“[Winning Game 3] gives us a little confidence, but you can’t take that for granted,” Pierce said. “The Heat is going to come out and play a different game [Monday]. We’re going to come out and try and do what we do and play the type of defense that we got to play. Games change night in and night out. You can’t predict what’s going to happen.

“It gives us a little confidence knowing that we can beat them, but we feel like we can beat anybody.”